Manufacture of selenium and like rectifiers



W 1! L. E. THQMIPSON 2335mm MANUFACTURE OF SELENIUM AND LIKE RECTIFIEBS Filed Jan; 31, $1.940

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i ainted Mar. 18, 1941 NrrEo STATES PATENT OFFICE MANUFACTURE OF SELENIUM AND LIKE RECTIFIERS Application January 31, 1940, Serial No. 316,608 In Great Britain May 26, 1939 2 Claims.

This invention relates to selenium and like rectillers oi the kind comprising a metallic base or support provided with a coating or layer composed oi or containing selenium or the like, superposed on which is a counter-electrode or contact layer composed oil a suitahle metal or alloy, the invention having for its ohject to provide an improved process oi manufacture oi rectihers of this character wherehy their production on a relatively large scale may he more conveniently and economically eilected than hy esisting processes.

has hitherto heen customary to manuiacture each individual rectiiler element separately as re the preparation at the metallic base and the application thereto oi the selenium and contact layers, this process, particularly in the case small rectiher elements, heing relatively slow and espenslve.

.hccording to the present invention, however, a hate or support oi suilicient site to provide a relatively large nuinher oi rectifier elements is arranged to he coated as a whole with the required layer oi selenium or the litre, and subjected to annealing or other treatment, the coated surface oi the support helng then coated with a contact metal over isolated areas each corresponding to a single rectiher element, the remaining portions oi the area oi the selenium coating heing protected hy means oi a suitable mash: during the application oi the metal coating and the coated hase thus i'ormed heing iinaily sch-divided lay cutting or otherwise at the portions oi the selenium coating thus protected.

in order that the nature oi the invention may clearly imderstood, a preicrred process oi manui'acture emhodying the invention will now he hriedy descrihed hy way oi errarriple with refer-- ence to the accompanying drawing, Figures l, a and h, oi which are plan views representing various stages in the manulactm'e oil a selenium rectilier in accordance with the invention.

"the hose or support oil which a portion is indicated at l in Figures i and d is constituted by a strip oi sheet iron approximately hilt" thiclr, the

" width oi the strip corresponding to the width oil a single rectifier element, tor example i the strip hcing as long as can he conveniently handled, ior emonple, approirimately 2d.

the upper stu'iace oi the metal strip i is first roughened hy euposing this surface to the action or a sand hlast, the strip being preferably arranged to he passed under a sand-blasting noz'zl hy means at a suitable conveyor.

"llhe metal strips sites this treatment may be stored until required, care being taken to keep their roughened surfaces iree from dust, grease and rust.

The next stage in the process of manufacture consists in distributing powdered selenium material evenly over the roughened surface of the 5 metal strip, this material being preiera-hly prepared as described in the specification oi my co pending application ior Letters Patent oi the United States, Serial No. 3l'ii,d3'l, tiled on Janu ary 3], 19th, for lvlanuiacture oi selenium recttlil hers. The distrihution oi the powdered material is preferably eiiected hy a device comprising a hopper containing the material and provided at its lower end with a distrihuting device compile" ing a rotating spindle or shalt having suitahle W slots or grooves in its surface which, during the rotation oi the spindle, transier powdered mate rial from the base oi the hopper to an outlet passage containing a sauce diaphragm through which the material falls in a shower or spray on to the suriace oi the metal strip as the latter passes underneath the outlet. The strip is prei erahly carried along underneath the distrlhuting device on a suitahle conveyor, the driving mech-= anism of which is geared to the mechanism drlving the rotating spindle, arrangements heing pro..- vided, ii desired, ior adjusting the relative speed of movement of the conveyor and the spindle so as to regulate the distribution oi the powdered material. W

The strip with its-powdered layer oi material then passes into a heated press, preierahly hy draulically operated, in which the powdered layer is subjected to a pressure oi lid to lhhh the. per sq. inch at a temperature oi apprordmately lhh C. ior at least i to l minutes.

its a result oi this treatment, the powdered materlai is converted into a relatively hard layer which is firmly adherent to the metal strip, and the latter may he stored, in trays ior errample, until required tor the iurther stages oi the process oi manufacture, special care heing talren to avoid injury to the coated suriace oi the strips.

The nest stage in the process is the anncaling oi the rectifier stoclr termed as shove dcscrihed w and for this purpose the coated strips are heated in a suitable annealing iurnace to a temperature oi approrrimately 205 C. to 215 U. tor a period oi from 8 minutes to til minutes, the period cl maintenance at this temperature being longer the lower the temperature.

The strips are then allowed to cool and the further stages of the process are carried out with as little delay as possible.

The first oi these further stages may consist in immersing the strips in a hot solution of dilute caustic soda, this solution being preferably prepared by dissolving V lb. caustic soda in a gallon of water, the temperatures of the soda bath being from C. to C. After removal from the caustic soda bath, the strips are washed in running water and allowed to drain and to dry. This caustic soda treatment is however, optional and may b omitted if desired.

A suitable mask is then applied to the coated surface of each strip, this mask being provided with a number of apertures each corresponding to the eifective area of a single rectifier element.

The strip attached to or assembled in the mask is then provided with a counter electrode composed of an alloy of tin and cadmium containing approximately from 25% to 60% of cadmium this alloy being applied by a spraying operation preferably eil'ected by means of a suitable metallisation pistol operated at a comparatively low pressure of approximately 30 lbs. per sq. inch.

The mask having been removed, the strip which then presents the appearance shown in Figure l or 3 in which the metal coating is indicated at 2, is sub-divided into individual rectifier elements by cutting or shearlng along the dotted lines indicated at a, b in Figures 1 and 3, thme lines being located in the selenium coated portions of the surface of the strip which, having been protected by the mask, are not coated with the metallic alloy. It will be observed that the portions of the strip between the lines, a, b are discarded as waste, the separated rectifier elements being of the larger size shown in Figure 2 or of the smaller size shown in Figure 4. Elements of still smaller size may of course be produced in a similar manner by further subdivision of the original strip.

When, as is usually the case, the rectifier elements are to be each provided with a central aperture for the purpose of assembly on a rod or bolt, the mask will be of such form as to protect central circular areas 3 for each of the elements eventually formed, and the necessary apertures l may be punched in these areas at the same time as the strip is sub-divided as above described.

The separated rectifier elements may then be subjected to an electrical forming treatment such for example as that described in the copending application for Letters Patent of the United States, Serial No. 309,040, filed by myself and Alexander Jenkins on December 13, 1939, for Alternating current rectifiers of the selenium typ the rectifier elements being finally tested for current in the forward and reverse directions.

It will be understood from the above description that during the earlier stages of the process of manufacture the strip or sheet comprising a large number of rectifier elements is a unit which can be readily handled and treated and it is found that the subsequent sub-division of the strip can be effected without appreciable injury to the selenium coating, while the parts of the surface provided with the superposed metallic layer or coating are unaifected by the cutting or shearing operation, since this is effected at the other portions of the surface.

The invention is evidently not limited to the particular process above described by way of example.

Having .now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

l. A process for the manufacture of selenium or like rectifiers of the kind described, in which a base or support of suiiicient size to provide a relatively large number of rectifier elements is arranged to be coated as a whole with the required layer of selenium or the like, and subjected to annealing or other treatment, the coated surface of the support being then coated with a contact metal or alloy over isolated areas each corresponding to a single rectifier element, the remaining portions of the area of the selenium coating being protected by means of a suitable mask during the application of the metal coating and the coated base thus formed being finally sub-divided. by cutting or otherwise at the portions of the selenium coating thus protected, for the purpose specified. 4

2. A process as claimed in claim 1, in which the final sub-division oi the base or support is effected by a double cutting or shearing operation, substantially as described. 

